Blends of thermoplastic materials are useful for various applications. U.S. Pat. No. 4,179,479 to Russell P. Carter, Jr. relates to thermoplastic polyurethane materials containing a processing aid. More particularly, the patent to Carter relates to a thermoplastic polyurethane composition comprising: (A) from 40 to 100 percent by weight of a thermoplastic polyurethane, (B) from 0 to 60 percent by weight of a thermoplastic polymer selected from the group consisting of thermoplastic polycarbonates, thermoplastic polyoxymethylenes, thermoplastic acrylonitrile/butadiene/styrene graft copolymers, thermoplastic polybutylene terephthalates, thermoplastic polyethylene terephthalates, and mixtures thereof and (C) from 0.5 to 10 percent by weight based on the amount of (A) and (B), of a processing aid which is an acrylic polymer having a number average molecular weight of from 500,000 to 1,500,000.
Although the patent discloses blends that include thermoplastic polymers with an acrylic processing aid, it is tellingly silent as to the electrostatic dissipative properties of the blends. Nonetheless, the accumulation and retention of static electrical charges on the surfaces of most plastics due to their low conductivity is well known. The accumulation of static charge on the surface of a plastic is undesirable for various reasons. Sometimes the static charge on these materials can discharge very quickly which can possibly damage components or articles which come in contact with the plastic. In addition, dust is typically attracted to and accumulates on materials carrying a static charge. Therefore, the electrostatic dissipative properties of plastic materials are of importance in various applications where static charge accumulation must be avoided.
Four major approaches have been used to alleviate the accumulation of static electrical charges in plastics: external chemical treatments, internal chemical additives, conductive fillers and polymeric additives. Each approach while effective in certain specific applications, suffers from deficiencies.
The external chemical treatments, hygroscopic surfactants which can be applied to the surface of the plastic article, suffer due to their reliance on high humidity for effectiveness. Generally, the relative humidity needs to be greater than 30%. Also, permanence is an issue since the external chemical treatment can be rubbed, wiped or washed off resulting in a loss of electrostatic dissipative ("ESD") protection. These types of chemical treatments are particularly undesirable in sensitive electronic handling applications where contamination of the components is an issue. Examples of these chemical treatments include fatty acids and their amines or salts, quaternary ammonium salts, monoalkyl glycerides, alkyl phosphonates and sulfonamides.
Internal chemical additives rely upon additive migration to the surface of the plastics to provide ESD protection to the plastic. Internal chemical additives are generally low molecular weight non-polymeric, hygroscopic surfactants which are compounded into the plastic material itself. While this approach provides more permanence than the external chemical treatments, just like external chemical treatments it is also prone to be rubbed, wiped or washed away. Internal chemical additives migrate to the surface of the plastic because of their limited compatibility with the plastic. When the additive migrates to the surface, it can be wiped, abraded or washed off, leaving the surface without any ESD protection. A static charge can then build on the surface, since the material is not protected. This lack of permanence results in periods of ESD susceptibility in which the plastic cannot dissipate a charge until additional additives can bloom to the surface. In addition, contamination of sensitive components which come into contact with the plastic article containing the ESD additive can occur. The internal chemical additives also depend upon high humidity to be effective.
The use of conductive fillers can also alter the ESD properties of plastics. Examples of conductive fillers include conductive carbon black and metallic fibers or fillers. Although the conductive fillers are neither humidity dependent nor susceptible to being wiped, washed or rubbed away, they do have certain disadvantages. For example, conductive fillers tend to increase the melt viscosity for processing the blend. Also, conductive fillers tend to limit the colorability of the plastic materials. Generally, they also tend to decrease the physical properties of the blend such as impact resistance.
Numerous examples of internal polymeric additives exist. An example of the addition of an electrostatic dissipative polymer being added to an insulative polymer can be found in U.S. Pat. No. 3,425,981 to Puletti. The patent to Puletti discloses an olefin polymer composition containing ethylene oxide based polymers and exhibiting enhanced antistatic properties. Additionally, U.S. Pat. No. 5,010,139 to Yu discloses an antistatic polymeric composition consisting of a blend of a polymeric material and antistatic additive which is an ethylene oxide based copolymer or terpolymer material. The polymeric material can be any thermoplastic, thermoplastic elastomer or elastomer including acrylonitrile butadiene styrene (ABS); copolymers of styrene and acrylonitrile modified with acrylic elastomers (ASA); polyamides; polybutylene terephthalate (PBT); polyethylene terephthalate (PET); polyethylene terephthalate glycol (PETG); polymethylmethacrylate (PMMA); polyurethane (TPU); polyvinyl chloride (PVC);chlorinated polyvinyl chloride (CPVC); polycarbonate (PC); polyoxymethylene (POM); polyphenylene oxide (PPO); copolymer of styrene and maleic anhydride SMA; and styrene acrylonitrile copolymer (SAN).
U.S. Pat. No. 5,159,053 discloses a thermoplastic polyurethane which has electrostatic dissipative properties. The thermoplastic polyurethane comprises the reaction product of an ethylene ether oligomer glycol reacted with a non-hindered diisocyanate and an extender glycol. The ethylene ether oligomer intermediate comprises a polyethylene glycol, having a average molecular weight from about 500 to about 5,000.
U.S. Pat. No. 5,342,889 discloses electrostatic dissipative polymeric compositions which are blends of an effective amount of a chain extended polymer and a matrix polymer. The chain extended polymer is formed from low molecular weight polyethers which are reacted with a chain extender and a diisocyanate. The matrix polymers include PVC, CPVC, a terpolymer of styrene, acrylonitrile and diene rubber; a copolymer of styrene and acrylonitrile modified with acrylate elastomers; a copolymer of styrene and acrylonitrile modified with ethylene propylene diene monomer rubber; rubber modified impact polystyrene; thermoplastic polyesters including PBT, PET and polyether-ester block copolymer; polyphenylene oxide; polyacetal; polymethyl methacrylate or mixtures thereof.
The usage of polymeric additives to alleviate the accumulation of static electrical charges in plastics provides a balance of performance advantages not achieved by the other ESD approaches described above. The polymeric additives are generally permanent, non-contaminating, colorable, easy to process, uniform in properties and even recyclable.
Notwithstanding these known polymeric blends and electrostatic dissipative agents, there exists a need for a blend comprising a thermoplastic base polymer and a polymeric additive having improved electrostatic dissipative properties sufficient to cause the bleeding off or dissipation of any occurring static charge.